Various standard electronic ballasts used in fluorescent lighting result in undesirable oscillation of the lamp current envelope when dimming to low levels of lamp current. Such oscillation of the lamp current envelope results in poor crest factor, and may cause flicker of the light and in some instances extinction of the arc.
In such standard lamp current control techniques of the prior art, current feedback has been utilized. The oscillation of the lamp current, i.e. moding or modulation of the amplitude, particularly affects narrow-tube lamps, especially at low current levels.
Various of the prior techniques used to alleviate moding are incomplete. For example, applicable current and frequency ranges are limited. Other potential prior solutions have contemplated fairly complex schemes. Such complex systems, however, involve significantly increased numbers of components and costs. For example, prior attempts for solving moding have involved about ten times the number of components at costs ranging from 10 to 20 times that proposed by the present invention.
Typical standard commercial fluorescent ballasts may be seen by reference to U.S. Pat. Nos. 4,952,849 and 5,089,751, both of which are assigned to the same assignee as the present application. Such prior techniques involve control circuitry that senses the lamp current, applies a R-C lowpass filter to give a low frequency roll-off pole, compares the signal to a reference, and adjusts the frequency of a half-bridge driver. If this type of electronic ballast is operated on a pair of series-connected quad tube fluorescent lamps, the arrangement results in lamp current moding which destabilizes the lamp current envelope at low current levels.
For purposes of this application a frequency response zero is as defined at page 1030, and a pole is as defined at page 660 of the IEEE Standard Dictionary of Electrical and Electronic Terms, 3rd Ed, 1984.